Conventionally, Japanese Patent Laid-Open Nos. 2000-292686 and 2001-305415 disclose techniques of imparting a phase-difference detection function to an image sensor to implement high-speed phase-difference detection type AF without using a dedicated AF sensor.
In Japanese Patent Laid-Open No. 2000-292686, each of the light-receiving units of some pixels in an image sensor is divided into two parts to impart a pupil division function, thereby forming focus detection pixels. Phase-difference detection type AF is implemented using the focus detection pixels arranged at a predetermined interval. Note that the focus detection pixels incapable of capturing an image are handled as defective pixels, and image information corresponding to them is generated by interpolating neighboring image-capturing pixel information.
Japanese Patent Laid-Open No. 2001-305415 discloses generating image information of focus detection pixels by adding up outputs from the two light-receiving units of each focus detection pixel, instead of interpolating neighboring image-capturing pixel information.
However, since the focus detection pixel in the Japanese Patent Laid-Open Nos. 2000-292686 and 2001-305415 has a light-receiving unit that is divided in two, the light-receiving efficiency lowers, resulting in a narrower dynamic range and a lower S/N ratio.
Japanese Patent Laid-Open No. 2000-156823 discloses a technique which uses some pairs of pixels in an image sensor as focus detection pixels by decentering the sensitive regions of light-receiving units in opposite directions with respect to the optical axes of on-chip microlenses. Phase-difference detection type AF is performed based on a pair of AF waveforms obtained from such a pair of focus detection pixels. In Japanese Patent Laid-Open No. 2000-156823, the image information of the focus detection pixels is generated by interpolating neighboring image-capturing pixel information, as in Japanese Patent Laid-Open No. 2000-292686.
Japanese Patent Laid-Open No. 2000-156823 can suppress degradation in the light-receiving efficiency of the focus detection pixels, but has the following problems.
Since each pair of focus detection pixels whose light-receiving units have sensitive regions decentered in the opposite directions includes different pixels, the individual pixels receive light fluxes from different portions of a subject image. Hence, image waveforms having a phase shift corresponding to the interval between the pixels included in the pair of focus detection pixels are obtained. The shift amount is not constant and is hard to correct because it changes depending on the pattern of the subject. This causes a focus detection error. Additionally, in Japanese Patent Laid-Open No. 2000-156823, since two pixels that form a pair of focus detection pixels are spaced apart from each other, a focus detection error occurs at higher probability.